The invention relates to an operating circuit for discharge lamps.
In this case, the invention relates to operating circuits which supply the discharge lamp with radio-frequency supply power which is obtained from a supply power via an oscillator circuit. In particular, but not necessarily, the invention relates to the situation where the supply power for the oscillator circuit is obtained from an AC voltage supply power which is rectified. Operating circuits such as these are in general use, in particular for low-pressure discharge lamps, and there is therefore no need to explain their details.
The oscillator circuit in this case supplies a so-called load circuit, in which the discharge lamp is connected, and through which a radio-frequency lamp current flows, which is produced by the oscillator circuit. The load circuit in this case defines a resonant frequency, which is influenced by various electrical parameters of the load circuit and also depends, inter alia, on the operating state of the discharge lamp. The aim is to operate the load circuit relatively close to the resonant frequency during continuous operation of the discharge lamp. This has the advantage of small phase shifts between the current and voltage, and hence of small reactive currents. This is beneficial for dimensioning of the components, particularly for a lamp inductor. Apart from this, the oscillator circuit which produces the radio-frequency supply power normally contains switching elements. When the phase shifts are low as a result of operation close to resonance, the switching losses in the switching elements are relatively small. This has advantages with regard to the efficiency of the operating circuit and with regard to the thermal load and the dimensioning of the switching elements.
Normally, one aim is to operate in the so-called inductive region, that is to say at an oscillator circuit operating frequency that is higher than the resonant frequency of the load circuit. However, in this case, it is necessary to avoid the operating frequency of the oscillator circuit becoming less than the resonant frequency since disturbing current spikes can be produced in the switching elements, and other difficulties can occur, in capacitive operation, that is to say when the operating frequency is less than the resonant frequency. In particular, incorrect synchronization between the switching times and the lamp inductor current during capacitive operation can lead to a pronounced positive current spike at the start of a lamp current half-cycle that is carried by a switching element. Thus, overall, it is desirable to operate as close as possible to the resonant frequency although, as far as possible, the frequency should not fall below the resonant frequency, or this should occur only to a restricted extent.
However, temperature changes and aging processes such as electrode wear, mercury diffusion in fluorescent substances and other aging phenomena as well as scatter between the individual examples of different individual discharge lamps result in fluctuations in the lamp impedance (with respect to continuous operation).
These lamp impedance fluctuations and the normal component tolerances mean that the operating circuits cannot easily be set relatively accurately to operation close to resonance. In fact, for safety reasons, a relatively large margin is maintained from the nominal resonant frequency, to take account of the fluctuations and tolerances as described. This results in higher component costs and an increased amount of space being required owing to correspondingly larger dimensioning and in reductions in efficiency.
Attempts have therefore already been made to equip operating circuits of the type described with detection circuits for identifying proximity to capacitive operation of the load circuit. By way of example, FIG. 5 in U.S. Pat. No. 6,331,755 illustrates a resistor RCS for measuring a lamp inductor current, and a comparator COMP for comparing this inductor current with a threshold value. The comparison is carried out on a switching-off flank of a switching transistor in a half-bridge oscillator circuit. The closer the operating frequency is to the resonant frequency and hence to capacitive operation, the smaller not only is a switching-on peak of the measurement voltage (at which the mathematical sign is reversed) across the resistor RCS, but the greater is the extent to which the measurement voltage falls, as well, at the end of the time for which said switching transistor is switched on. The threshold value therefore allows a limit state to be set, at which the circuit is switched off overall (shown on the right in FIG. 6 in that document), when operation becomes too close to resonance.
Against the background of the cited prior art, the invention is based on the technical problem of further improving an operating circuit for a discharge lamp having an oscillator circuit and having a detection circuit for identifying proximity to capacitive operation of the load circuit.
The invention relates to an operating circuit of the described type, in which a regulation circuit is provided for regulating the load circuit, in particular the lamp power or the lamp current, to a nominal regulation value, and the operating circuit is designed to reduce the nominal regulation value in response to the detection circuit identifying proximity to capacitive operation.
Preferred embodiments are specified in the dependent claims.
According to the invention, the operating circuit is not switched off, as in the case of the prior art, when specific proximity to capacitive operation is identified but, at least normally, is still operated. Identification of proximity to capacitive operation is thus intended to lead to the method of operation being influenced such that this proximity is at least not increased any further, or is even reduced, in order to allow operation to continue. For this purpose, the nominal regulation value, that is to say by way of example the nominal power or current value, of a regulation circuit is reduced. The regulation circuit intrinsically has the purpose and advantage of reducing the influence on lamp operation of scatter between individual lamps and fluctuations which occur over time, such as temperature fluctuations or aging influences. In the invention, a regulation circuit furthermore offers a particularly advantageous and simple capability to prevent capacitive operation by influencing the nominal regulation value. In one preferred embodiment of the regulation circuit, changing the nominal regulation value can also be associated with indirectly influencing the operating frequency of the oscillator circuit, because the regulation circuit preferably influences the operating frequency, in order to regulate the load circuit. In plain words, the operating circuit according to the invention is thus designed not to excessively approach capacitive operation during continuous operation and to counteract any further approach if it becomes too close, but with lamp operation continuing. This is because it is more tolerable from the point of view of the invention for the discharge lamp to become slightly darker in situations such as this than for it to be switched off entirely.
The invention is preferably distinguished by the detection circuit identifying proximity to capacitive operation in a particularly advantageous form. To do this, the detection circuit detects the magnitude of fluctuations of the lamp current corresponding to the frequency of the supply power. If the oscillator circuit is supplied with a rectified AC supply power, the supply power of the oscillator circuit fluctuates with the fluctuations (which result from the AC frequency) of the rectified supply voltage (so-called intermediate circuit voltage). The intermediate circuit voltage is thus modulated at twice the frequency of the original AC voltage. The doubling of the frequency is a consequence of the rectification process. Theoretically, it is also feasible in this case for no frequency doubling to occur; in any case, the modulation of the intermediate circuit voltage is related to the frequency of the original AC voltage.
This intermediate circuit voltage modulation can generally still be measured in the lamp current itself, to be precise even when the lamp current is regulated by means of a current or power regulation circuit. Depending on the technical complexity, regulation circuits are able to attenuate this modulation only to a limited extent.
Incidentally, this is also true in the situation, which represents one preferred embodiment of the invention, in which the rectified AC supply power is converted to a largely constant DC voltage by means of a power factor correction circuit (PFC circuit). The PFC circuit is used to limit the harmonic content of the power consumption from the AC voltage network, and generally charges an energy storage capacitor to the intermediate circuit DC voltage. The intermediate circuit voltage is also then modulated to a certain extent on the basis of the AC voltage frequency.
The magnitude of the lamp current fluctuations depends on the proximity to the resonant frequency and hence on the proximity to capacitive operation. This follows from the increase in the lamp current with increasing proximity to resonance on the one hand, and from the modulation of the proximity to resonance by the intermediate circuit voltage modulation, on the other hand.
The magnitude of the fluctuations of the lamp current thus offers a particularly simple possible way to detect proximity to capacitive operation. In particular, this relates to a signal which varies, for example, at twice the mains frequency of the AC voltage network, and which to this extent does not represent any significant measurement difficulties. On the other hand, the conventional solutions for detecting proximity to capacitive operation are linked to the operating frequency of the oscillator circuit itself and must be related to these phases, which involves a considerably greater degree of circuitry complexity. In the case of the invention, the lamp current has to be measured in any case, in order to carry out the current regulation that has already been mentioned. Thus, overall, the invention is associated with less additional complexity.
The description here has referred in general to a variable supply power. As stated above, this may on the one hand be a rectified AC supply power. However, the invention also covers the situation where the operating circuit is operated from a DC voltage source. There is then no need for a rectifier, or any rectifier which is provided in any case has no effect. However, even in this case, it may be desirable to use the invention. The DC voltage or intermediate circuit voltage may be deliberately modulated for this purpose. In addition to the capability for detection according to the invention of the proximity to capacitive load circuit operation, this furthermore has the advantage that the modulation results in a broadening of the frequency spectrum of radio-frequency interference which is transmitted through the operating circuit to the DC voltage source. The interference is thus less problematic because it occurs over a wider, and hence flatter, interference spectrum. Thus, for the purposes of the claims, the variable supply powers may also be deliberately modulated DC supply powers. In particular, the invention also relates to combination operating circuits which are intended for operation from both DC and AC voltage sources.
Furthermore, the invention alternatively relates to detection of the magnitude of fluctuations of the lamp current itself even in a situation where the lamp current is governed by a regulation circuit for regulating the load circuit, that is to say in particular the lamp current or the lamp power, with a manipulated variable for the regulation circuit then being detected, that is to say the changes in the regulation circuit while the regulation circuit is trying to stabilize the controlled variable. The manipulated variable could then be regarded as an image of the lamp current fluctuations, even when the latter are not occurring, or are occurring only to a minor extent.
The regulation circuit preferably has an I regulation element, that is to say an integrating element, in order to compensate for the comparatively slow parameter changes in the discharge lamp in the sense of the described impedance changes caused by aging or other long-term fluctuations. An I regulation element such as this will be sufficient in many cases. If required, it may be supplemented by a P regulation element (proportional element) or by some other additional device in order to take better account of the intermediate circuit voltage modulation.
In particular, it is possible to provide for the detection circuit to compare the magnitude of the fluctuations with a predetermined threshold value and not to influence operation any further unless the threshold value is exceeded. If the threshold value is exceeded, the detection circuit can either continuously vary the nominal regulation value in accordance with a regulation context, or else can vary it by a predetermined fixed amount, as is described in the exemplary embodiment. In any case, the comparison with the threshold value preferably results in a detection circuit function which does not influence operation in normal circumstances.
In particular, the regulation circuit and any other control of the oscillator circuit can be provided by means of an integrated digital circuit which need have only a small number of additional functions. Furthermore, the digital circuit may be a programmable circuit or a so-called microcontroller, in which case the additional complexity that is required for the invention can be restricted just to additional software.
A digital control circuit such as this or a microcontroller such as this may also, in particular, control the PFC circuit that has been mentioned, in addition to controlling the oscillator circuit.